Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
HIGH TENSILE STRENGTH HOT-DIPPED STEEL SHEET
AND METHOD OF PRODUCING THE SAME
TECHNICAL FIELD
This invention relates to a high tensile strength hot-dipped steel sheet
usable for a vehicle body of an automobile or the like formed by subjecting a
surface of a high tensile strength steel sheet to a hot dipping of zinc
(including an
alloy thereof, the same is applied hereinafter), aluminum, zinc-aluminum
alloy,
zinc-aluminum-magnesium alloy or the like, and a method of producing the same.
BACKGROUND ART
Recently, the application of high tensile strength hot-dipped steel
sheets formed by subjecting a surface of a steel sheet to galvanizing or the
like is
increasing as a steel sheet for an automobile from viewpoints of safety,
weight
reduction and low fuel consumption of the automobile and hence global
environment protection.
In order to obtain such a high tensile strength hot-dipped steel sheet,
it is important to use a steel sheet having an excellent plating property and
providing desired strength and workability (press formability or the like)
after
being passed through a hot dipping bath or further subjected to an alloying
treatment as an original sheet.
In general, Si, Mn and so on are added to the steel sheet for increasing
the strength of the steel sheet. However, it is known that when the steel
sheet
added with such elements is subjected to a plating in, for example, a
continuous
galvanizing line (CGL), the plating property is degraded because oxides of Si,
Mn and so on are formed on the surface of the steel sheet at an annealing step
before the plating.
This phenomenon is caused due to the fact that when the annealing is
carried out in a reducing atmosphere before the plating, since such an
atmosphere
is reducing for Fe but is oxidative for Si, Mn and the like in steel, Si, Mn
and the
like are selectively oxidized on the surface of the steel sheet to form
oxides.
Since such surface oxides considerably lower a wettability of fused
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zinc to the steel sheet, the plating property is degraded in the galvanized
steel
sheet using a high tensile strength steel sheet as an original plating sheet,
and
particularly, when the contents of Si, Mn and the like are high, there is a
problem
that the plating is not locally conducted or so-called non-plated portion is
formed.
As means for improving the degradation of the plating property in
such a high tensile strength steel sheet, for example, JP-A-55-122865 and JP-A-
9-13147 propose a method of forcedly oxidizing the steel sheet under a high
oxygen partial pressure and then reducing it prior to the heating during the
plating. And also, a method of conducting a preliminary plating before the hot
dipping is proposed in JP-A-58-104163.
However, the former method has problems that the control of the
surface oxide through forcible oxidation is not sufficiently carried out and
the
stable plating property is not necessarily guaranteed in accordance with compo-
nents in steel and plating conditions. On the other hand, the latter method
has a
problem that the production cost rises because an extra process should be
added.
Besides, JP-A-6-287684 discloses a high strength steel sheet having
an improved plating property by optimizing addition amounts of P, Si and Mn.
And also, JP-A-7-70723 and JP-A-8-85858 propose a method wherein a
recrystallization annealing is previously carried out before the plating to
form a
surface oxide and then a galvanizing is carried out after such an oxide is
removed
by pickling.
By these method could be prevented the occurrence of the non-plated
portion in a substantial quantity of high-strength steels.
Even in these methods, however, there is still a problem that the
occurrence of the non-plated portion can not be completely prevented as to a
type
of steel having a higher Si content.
DISCLOSURE OF THE INVENTION
It is an object of the invention to advantageously solve the afore-
mentioned problems and to propose a high tensile strength hot-dipped steel
sheet
capable of effectively preventing the occurrence of non-plated portions even
if a
high tensile strength steel sheet having higher contents of Si and Mn is used
as an
original plating sheet as well as a production method usable therefor.
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The inventors have made various studies in order to solve the above
problems and obtained a knowledge that a) as components, Nb and Cu or Ni, Mn
are compositively added while regulating Si content to a given range, b) an
internal oxide layer is formed just beneath a surface of a steel sheet through
an
annealing in a continuous annealing line (CAL) (hereinafter referred to as a
recrystallization annealing) and a simultaneously formed surface oxide is
removed
by picking after the annealing, c) in a subsequent heating before a plating in
a
continuous galvanizing line (CGL)(hereinafter referred to as a heating before
plating), the formation of oxides of Si, Mn and the like is considerably
decreased
on the surface of the steel sheet as the above internal oxide layer acts as a
diffusion barrier, and hence a big improvement of the plating property can be
attained.
The invention is accomplished based on the above knowledge.
In a broad aspect, the present invention relates to a high tensile strength
hot-dipped steel sheet, characterized in that the
hot-dipped steel sheet is obtained by subjecting a steel sheet of a
composition
comprising:
C: not more than 0.010 mass%,
Nb: not less than 0.005 mass % and not more than 0.2 mass %,
not less than 0.03 mass % and not more than 1.5 mass % in total of one or
more selected from Cu: less than 0.5 mass%, Ni: less than 1.0 mass % and Mo:
less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: not more than 0.010 mass %, and containing Si: not less than 0.25
mass % and not more than 1.2 mass %,
Mn: not less than 0.50 mass % and not more than 3.0 mass % in a range
satisfying 1.5.xSi(mass %)<Mn(mass %),
and the remainder being Fe and inevitable impurities to a recrystallization
annealing and forming an internal oxide layer in a reducing atmosphere having
a
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dew point of not higher than 0 C and not lower than -45 C at an annealing
temperature of not lower than 750 C, cooling, removing oxides formed on a
surface of the steel sheet by pickling, reheating to a temperature of not
lower than
650 C and not higher than 850 C in a reducing atmosphere having a dew point
of
not higher than -20 C, and subjecting to a hot-dipping treatment during the
course
of cooling down from the reheating temperature to provide a hot-dipping layer
on
the surface of the steel sheet.
In another broad aspect, the present invention relates to a high tensile
strength hot-dipped steel sheet, characterized in that the
hot-dipped steel sheet is obtained by subjecting a steel sheet of a
composition
comprising:
C: not less than 0.03 mass % and not more than 0.20 mass %,
Nb: not less than 0.005 mass % and not more than 0.2 mass %,
not less than 0.03 mass % and not more than 1.5 mass % in total of one or
more selected from Cu: less than 0.5 mass %, Ni: less than 1.0 mass % and Mo:
less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: not more than 0.010 mass %,
Si: not less than 0.5 mass % and not more than 1.5 mass %,
Mn: not less than 1.2 mass % and not more than 3.5 mass % in a range
satisfying 1.5xSi(mass %)<Mn(mass %), at least one of Ti and V in a range
satisfying total of Ti and
V: not more than 0.5 mass % and Ti(mass %)<5xC(mass %),
and the remainder being Fe and inevitable impurities to a recrystallization
annealing and forming an internal oxide layer in a reducing atmosphere having
a
dew point of not higher than 0 C and not lower than -45 C at an annealing
temperature of not lower than 750 C, cooling, removing oxides formed on a
surface of the steel sheet by pickling, reheating to a temperature of not
lower than
650 C and not higher than 850 C in a reducing atmosphere having a dew point of
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not higher than -20 C, and subjecting to a hot-dipping treatment during the
course
of cooling down from the reheating temperature to provide a hot-dipping layer
on
the surface of the steel sheet.
In another broad aspect, the present invention relates to a high tensile
strength hot-dipped steel sheet, characterized in that the
hot-dipped steel sheet is obtained by subjecting a steel sheet of a
composition
comprising:
C: not less than 0.03 mass % and not more than 0.20 mass %
Nb: not less than 0.005 mass % and not more than 0.2 mass %,
not less than 0.03 mass % and not more than 1.5 mass % in total of one or
more selected from Cu: less than 0.5 mass %, Ni: less than .1.0 mass % and Mo:
less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: not more than 0.010 mass %,
Si: not less than 0.5 mass % and not more than 1.5 mass %,
Mn: not less than 1.2 mass % and not more than 3.5 mass % in a range
satisfying 1.5xSi(mass %)<Mn(mass %),
at least one of Ti and V in a range satisfying total of Ti and
V: not more than 0.5 mass % and Ti(mass %)<5xC(mass %),
Cr: not more than 0.25 mass % and satisfying Si(mass %)>3xCr(mass %),
and the remainder being Fe and inevitable impurities to a recrysallization
annealing and forming an internal oxide layer in a reducing atmosphere having
a
dew point of not higher than 0 C and not lower than -45 C at an annealing
temperature of not lower than 750 C, cooling, removing oxides formed on a
surface of the steel sheet by pickling, reheating to a temperature of not
lower than
650 C and not higher than 850 C in a reducing atmosphere having a dew point of
not higher than -20 C, and subjecting to a hot-dipping treatment during the
course
of cooling down from the reheating temperature to provide a hot-dipping layer
on
the surface of the steel sheet.
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In another broad aspect, the present invention relates to a high tensile
strength hot-dipped steel sheet, characterized in that the
hot-dipped steel sheet is obtained by subjecting a steel sheet of a
composition
comprising:
C: not less than 0.03 mass % and not more than 0.20 mass %,
Nb: not less than 0.005 mass % and not more than 0.2 mass %,
not less than 0.03 mass % and not more than 1.5 mass % in total of one or
more selected from Cu: less than 0.5 mass %, Ni: less than 1.0 mass % and Mo:
less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: not more than 0.010 mass %,
Si: not less than 0.5 mass % and not more than 1.5 mass %,
Mn: not less than 1.2 mass % and not more than 3.5 mass % in a range
satisfying 1.5xSi(mass %)<Mn(mass %),
Cr: not more than 0.25 mass % and satisfying Si(mass %)>3xCr(mass %),
and the remainder being Fe and inevitable impurities to a recrystallization
annealing and forming an internal oxide layer in a reducing atmosphere having
a
dew point of not higher than 0 C, and not lower than -45 C at an annealing
temperature of not lower than 750 C, cooling, removing oxides formed on a
surface of the steel sheet by pickling, reheating to a temperature of not
lower than
650 C and not higher than 850 C in a reducing atmosphere having a dew point
of
not higher than -20 C, and subjecting to a hot-dipping treatment during the
course
of cooling down from the reheating temperature to provide a hot-dipping layer
on
the surface of the steel sheet.
In another broad aspect, the present invention relates to a method of
producing a high tensile strength hot-dipped steel sheet,
characterized in that a steel sheet of a composition comprising:
C: not more than 0.010 mass%,
Nb: not less than 0.005 mass % and not more than 0.2 mass %,
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not less than 0.03 mass % and not more than 1.5 mass % in total of one or
more selected from Cu: less than 0.5 mass %, Ni: less than 1.0 mass % and Mo:
less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: not more than 0.010 mass %,
and containing Si: not less than 0.25 mass % and not more than 1.2 mass
Mn: not less than 0.50 mass % and not more than 3.0 mass % in a rang
satisfying 1.5 xSi(mass %)<Mn(mass %),
and the remainder being Fe and inevitable impurities is subjected to a
recrystallization annealing in a reducing atmosphere having a dew point of not
higher than 0 C and not lower than -45 C at an annealing temperature of not
lower than 750 C and cooled, and oxides formed on a surface of the steel sheet
are removed by pickling, and the steel sheet is reheated to a temperature of
not
lower than 650 C and not higher than 850 C in a reducing atmosphere having a
dew point of not higher than -20 C, and subjected to a hot-dipping treatment
during the course of cooling down from the reheating temperature.
The invention mainly lies in a feature that Nb and Cu or Ni, Mn are
compositively added while appropriating Si content, and an internal oxide
layer is
formed just beneath a surface of a steel sheet in the recrystallization
annealing,
and surface oxides simultaneously formed on the surface of a steel sheet are
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removed by pickling and then the steel sheet is subjected to the heating
before
plating and further to a hot dipping.
The reason why the composition range and the production conditions
of the recrystallization annealing, heating before plating and the like
according to
the invention are limited to the above ranges will be described below.
In the invention, the range of C content is divided into two regions,
whereby there can be obtained a high tensile strength hot-dipped steel sheet
having a tensile strength of 400-600 MPa grade and an excellent ductility and
a
high tensile strength hot-dipped steel sheet wherein the ductility is somewhat
lowered and the tensile strength is as very high as 500-1200 MPa grade.
At first, the invention is described with respect to the high tensile
strength hot-dipped steel sheet having a tensile strength of 400-600 MPa
grade.
In this high tensile strength hot-dipped steel sheet, it is required to limit
C
content and each content of Si, Mn, Ti and B to the following ranges.
C: not more than 0.010 mass%
It is desired to decrease C content for improving elongation and r-
value of the steel sheet. Particularly, when the C content exceeds 0.010
mass%,
even if proper contents of Ti and Nb are added, the effect of improving
properties
(particularly press formability) through these elements is not obtained, so
that the
C content is limited to not more than 0.010 mass%. Moreover, when the content
is less than 0.001 mass%, it is difficult to form an internal oxide layer
during the
recrystallization annealing, so that the C content is favorable to be not less
than
0.001 mass%.
Si: not less than 0.25 mass%, not more than 1.2 mass%
Si is an element effective for strengthening steel. Heretofore, it was
required to decrease Si content as far as possible so as not to form Si oxide
on the
surface of the, steel sheet in the heating before plating. In the invention,
however, even if Si is added in an amount of not less than 0.25 mass%, Nb and
Cu or Ni, Mo are compositively added to form an internal oxide layer of Si and
Mn just beneath the surface of the steel sheet in the recrystallization
annealing,
which controls the formation of oxides of Si and Mn on the surface of the
steel
sheet in the subsequent heating before plating, so that the steels according
to the
invention indicate a good plating property. Moreover, this mechanism is
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considered due to the fact that the internal oxide layer acts as a diffusion
barrier
against the movement of Si and Mn in steel to the surface of the steel sheet.
The above effect is never obtained unless Si should be added in an
amount of not less than 0.25 mass%. On the other hand, when the Si content
exceeds 1.2 mass%, SiO2 is formed on the surface of the steel sheet in the re-
crystallization annealing and such a surface oxide can not be completely
removed
at a subsequent pickling step and a part thereof is retained to create a non-
plated
portion. Therefore, the Si content is limited to a range of 0.25-1.2 mass%.
1.5 x Si(mass%) < Mn(mass%)
When the Si content is an amount satisfying a relationship of 1.5 x
Si(mass%) >_ Mn(mass%) in view of Mn content mentioned later, SiO2 is also
formed on the surface of the steel sheet in the recrystallization annealing
and
such a surface oxide can not be completely removed at the subsequent pickling
step and hence the non-plated portion is created.
Therefore, it is important that Si is added in a range of 0.25-
1.2 mass% and a range satisfying a relationship of 1.5 x Si(mass%) <
Mn(mass%),
respectively.
Mn: not less than 0.50 mass%, not more than 3.0 mass%
Mn contributes to enhance the strength but also has an effect of
controlling the formation of SiO2 on the surface of the steel sheet in the
recrystallization annealing to form a composite oxide of Si and Mn capable of
easily removing by pickling. However, when the Mn content is less than
0.50 mass%, the above effect is poor, while when it exceeds 3.0 mass%, Mn
oxide is formed on the surface of the steel sheet in the heating before
plating to
easily create a non-plated portion and also steel is too hardened to hardly
conduct
cold rolling. Therefore, the Mn content is limited to a range of 0.50-3.0
mass%.
Ti: not more than 0.030 mass%
Ti is added, if necessary, because it forms a carbide, a nitride or the
like to effectively contribute to the improvement of the workability of steel.
However, when Ti is excessively added, surface oxides of Si and Mn formed in
the recrystallization annealing become larger and hence it is difficult to
remove
such oxides by pickling. Therefore, the Ti content is limited to not more than
0.030 mass%. Moreover, Ti is not necessarily added.
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B: not more than 0.005 mass%
B is an element effective for improving a resistance to secondary
work brittleness. However, when B is added in an amount exceeding
0.005 mass%, the effect is not expected over a certain level but is rather
degraded
in accordance with the annealing conditions. And also, when B is excessively
added, hot ductility is lowered. Therefore, B is added in an amount of
0.005 mass% as an upper limit. Moreover, the B content is not particularly
critical with respect to the lower limit, but is sufficient to be added in
accordance
with an improving degree of the required resistance to secondary work
brittleness
and is desirable to be usually added in an amount of not less than 0.0010
mass%.
Next, the invention is described with respect to the high tensile
strength hot-dipped steel sheet having a tensile strength of 500-1200 MPa
grade.
In this high tensile strength hot-dipped steel sheet, it is required to limit
C
content and each content of Si and Mn to the following ranges.
C: not less than 0.03 mass%, not more than 0.20 mass%
C is an important, basic component in steel and is an element
contributing not only to improve the strength through bainite phase or
martensite
phase produced at a low temperature but also to precipitate carbides of Nb,
Ti, V
and the like to increase the strength. When the C content is less than 0.03
mass%,
not only the above precipitates but also bainite phase and martensite phase
are
hardly produced, while when it exceeds 0.20 mass%, a spot weldability is
degraded, so that the addition range is rendered into 0.03-0.20 mass%.
Moreover, a preferable C content is 0.05-0.15 mass%.
Si: not less than 0.5 mass%, not more than 1.5 mass%
Si is an element that the C content solid-soluted in a phase is
decreased to improve workabilities such as elongation and the like.
Heretofore,
it was required to decrease Si content as far as possible so as not to form Si
oxide
on the surface of the steel sheet in the heating before plating. In the
invention,
however, even if Si is added in an amount of not less than 0.5 mass%, Nb and
Cu
or Ni, Mo are compositively added to form an internal oxide layer of Si and Mn
just beneath the surface of the steel sheet in the recrystallization
annealing, which
controls the formation of oxides of Si and Mn on the surface of the steel
sheet in
the subsequent heating before plating, so that the steels according to the
invention
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indicate a good plating property. Moreover, this mechanism is considered due
to the fact that the internal oxide layer acts as a diffusion barrier against-
the
movement of Si and Mn in steel to the surface of the steel sheet.
The above effect is never obtained unless Si should be added in an
amount of not less than 0.5 mass%. On the other hand, when the C content is
0.03-0.20 mass%, if the Si content exceeds 1.5 mass%, Si02 is formed on the
surface of the steel sheet in the recrystallization annealing and such a
surface
oxide can not be completely removed at a subsequent pickling step and a part
thereof is retained to create a non-plated portion. Therefore, the Si content
is
limited to a range of 0.5-1.5 mass%.
Moreover, in order to control the occurrence of the non-plated portion
even in the steel sheet of 500-1200 MPa grade, the Si content is required to
control to a range satisfying 1.5 x Si(mass%) < Mn(mass%) in view of Mn
content mentioned later likewise the aforementioned case of the steel sheet of
400-600 MPa grade.
Mn: not less than 1.2 mass%, not more than 3.5 mass%
Mn has an effect of enriching 7-phase to promote martensite trans-
formation. And also, Mn has an effect that the formation of Si02 on the
surface
of the steel sheet in the recrystallization annealing is controlled to form a
composite oxide of Si and Mn capable of easily removing by pickling. However,
when the Mn content is less than 1.2 mass%, the effect is not obtained, while
when it exceeds 3.5 mass%, the spot weldability and plating property are
considerably damaged. Therefore, the Mn content is limited to a range of 1.2-
3.5 mass%, preferably 1.4-3.0 mass%.
Although the above is described with respect to the reasons on the
limited ranges of the respective components inherent to the steel sheets
having a
tensile strength of 400-600 MPa grade and the steel sheets having a tensile
strength of 500-1200 MPa grade, the following elements are required to be
added
as a component common to both kinds of the steel sheets.
Nb: not less than 0.005 mass%, not more than 0.2 mass%
Nb contributes to improve the plating property by making small a
crystal grain of the steel sheet produced in the recrystallization annealing
to
promote the formation of the internal oxide layer of Si and Mn just beneath
the
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surface of the steel sheet. The effect is not obtained unless Nb should be
added
in an amount of not less than 0.005 mass%. On the other hand, when the Nb
content exceeds 0.2 mass%, steel is hardened and hence the hot rolling or the
cold rolling is difficult but also the recrystallization annealing is
difficult because
the recrystallizing temperature is raised and a surface defect is caused.
Therefore, the Nb content is limited to a range of 0.005-0.2 mass%.
Not less than 0.03 mass% but not more than 1.5 mass% of one of or more in
total
of Cu: less than 0.5 mass%, Ni: less than 1.0 mass% and Mo: less than
1.0 mass%
Cu, Ni and Mo promote the formation of the internal oxide layer of Si
and Mn just beneath the surface of the steel sheet in the recrystallization
annealing, which controls the formation of oxides of Si and Mn on the surface
of
the steel sheet in the heating before plating, so that the steels according to
the
invention indicate a good plating property. This effect is not obtained unless
one or more of these elements should be added in an amount in total of not
less
than 0.03 mass%. On the other hand, when the content in total of these element
exceeds 1.5 mass%, or if the Cu content is not less than 0.5 mass%, the Ni
content is not less than 1.0 mass% and the Mo content is not less than 1.0
mass%,
the surface properties of the hot rolled sheet are degraded. Therefore, these
elements are added in amounts of Cu: less than 0.5 mass%, Ni: less than
1.0 mass%, Mo: less than 1.0 mass% and total amount of not less than
0.03 mass% but not more than 1.5 mass%.
Al: not more than 0.10 mass%
Al serves as a deoxidizing agent at a steel-making stage but also is
useful as an element for fixation of N causing aging degradation as A1N.
However, when the Al content exceeds 0.10 mass%, not only the rise of the
production cost but also the degradation of the surface properties are caused,
so
that Al is added in an amount of not more than 0.10 mass%. Preferably, it is
not
more than 0.050 mass%. Moreover, when the Al content is less than 0.005 mass%,
it is difficult to obtain the sufficient deoxidizing effect, so that the lower
limit of
Al content is favorable to be 0.005 mass%.
P: not more than 0.100 mass%
By adding P is increased the strength. However, when the P content
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exceeds 0.100 mass%, the segregation in the solidification becomes very
conspicuous and hence the increase of the strength is saturated and the
degradation of the workability is caused and further the resistance to
secondary
work brittleness is largely degraded and the steel is not substantially
durable in
use. Therefore, the P content is limited to not more than 0.100 mass%. In case
of an alloying galvanization, the P content is favorable to be not more than
0.060 mass% because it brings about the delay of the alloying. However, when
the P content is rendered into less than 0.001 mass%, the cost becomes too
much,
so that it is good to be not less than 0.001 mass%.
S: not more than 0.010 mass%
S causes a hot tearing in the hot rolling and induces a breakage of, a
nugget in a spot welded portion, so that it is desirable to decrease the S
content as
far as possible. And also, S causes an alloying unevenness in the alloying
treatment after the galvanization, so that it is also desirable to decrease as
far as
possible from this viewpoint. Further, the decrease of the S content
contributes
to the improvement of the workability through the decrease of S precipitates
in
steel and the increase of Ti content effective for fixing C. Therefore, the S
content is limited to not more than 0.010 mass%. More preferably, it is not
more than 0.005 mass%.
N: not more than 0.010 mass%
N is desirable to decrease as far as possible for ensuring properties
such as ductility, r-value and the like. Particularly, when the N content is
not
more than 0.010 mass%, a satisfactory effect is obtained, so that the upper
limit
is 0.010 mass%. Preferably, it is not more than 0.0050 mass%. Nevertheless,
the control of the N content to less than 0.0005 mass% brings about the rise
of
the cost, so that the lower limit is favorable to be 0.0005 mass%.
Although the invention is described with respect to the essential
components, when the C content is not less than 0.03 mass% but not more than
0.20 mass%, the following elements may be further added properly.
Ti and/or V: not more than 0.5 mass% under a condition satisfying Ti(mass%) <
x C(mass%)
Ti and V are elements forming carbides to render the steel into a
higher strength. However, when these elements are added in an amount
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exceeding 0.5 mass%, a disadvantage is brought in view of the cost and also
fine
precipitates become too large to obstruct recovery-recrystallization after the
cold
rolling and degrade the ductility (elongation). Therefore, even when these
elements are used alone or in a combination, they are added in an amount of
not
more than 0.5 mass%. More preferably, the content is 0.005-0.20 mass%.
However, when Ti is added in a range of Ti(mass%) >_ 5 x C(mass%),
the Ti content not forming the carbide increases, which is a cause of
degrading
the plating property, so that Ti is required to be added in a range satisfying
Ti(mass%) < 5 x C(mass%).
Cr: not more than 0.25 mass% under a condition satisfying Si(mass%) > 3 x
Cr(mass%)
Cr is an element effective for obtaining a composite structure of
ferrite + martensite likewise Mn, but when the Cr content exceeds 0.25 mass%
or
is Si(mass%) <_ 3 x Cr(mass%), Cr oxide is formed on the surface of the steel
sheet in the heating before plating to form a non-plated portion, so that the
Cr
content is limited to not more than 0.25 mass% under a condition satisfying
Si(mass%) > 3 x Cr(mass%). More preferably, it is not more than 0.20 mass%.
Moreover, the reason why the C content according to the invention is
"C: not more than 0.010 mass%" or "C: not less than 0.03 mass% but not more
than 0.20 mass" but excludes a range of "C: more than 0.010 mass% but less
than
0.03 mass%" is due to the fact that when the C content is within the above
excluded range, there is not obtained a product having a particularly
excellent
property with respect to the strength or workability.
Then, the invention is described with respect to reasons why the
recrystallization annealing conditions and the heating conditions before
plating
are limited to the above ranges.
Moreover, in the production method of the hot-dipped steel sheets
according to the invention, steps up to the recrystallization annealing, i.e.
hot
rolling step and cold rolling step are not particularly restricted, and these
steps
may be conducted according to usual manner.
Recrystallization annealing
The recrystallization annealing is carried out by heating to a
recrystallizing temperature (usually using CAL) for releasing strain
introduced in
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the cold rolling to provide mechanical properties and workability required for
the
steel sheet and forming the internal oxide layer of Si and Mn just beneath the
surface of the steel sheet.
Because, when such an internal oxide layer is existent, the formation
of oxides of Si and Mn is not caused on the surface of the steel sheet at the
subsequent heating before plating and the occurrence of the non-plated portion
is
controlled.
When the recrystallization annealing is carried out below 750 C, the
formation of the internal oxide layer is insufficient and the good plating
property
is not expected, so that it is necessary to conduct the recrystallization
annealing
above 750 C.
And also, the recrystallization annealing is necessary to be carried out
in a reducing atmosphere having a dew point of not higher than 0 C but not
lower than -45 C. Because, when the dew point is higher than 0 C, the oxide is
mainly Fe oxide and the internal oxide layer of Si and Mn is hardly formed,
while when the dew point is lower than -45 C, oxygen quantity is lacking and
the
internal oxide layer of Si and Mn is hardly formed. As the reducing
atmosphere,
nitrogen gas, argon gas, hydrogen gas and carbon monoxide gas may be used
alone or in an admixture of two or more gases.
Moreover, a temperature history of the recrystallization annealing is
preferable to be a pattern that the temperature is kept at 800-900 C for
0-120 seconds and then cooled at a rate of about 1-100 C/s.
Removal of surface oxide by pickling
The pickling is carried out for removing the oxides of Si and Mn
formed on the surface of the steel sheet in the reducing atmosphere by the
recrystallization annealing. As a pickling solution, it is favorable to use
3-30 mass% hydrochloric acid. And also, the pickling time is favorable to be
about 3-60 seconds.
Heating before plating
The heating before plating is carried out after the oxides of Si and Mn
are removed from the surface of the steel sheet by pickling. In the heating
before plating, it is preferable to usually use CGL. And also, the heating
before
plating is carried out in a reducing atmosphere having a dew point of not
higher
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than -20 C at a temperature of not lower than 650 C but not higher than 850 C.
Because, when the dew point of the atmosphere is higher than -20 C,
a thick Fe oxide is formed on the surface of the steel sheet to bring about
the
degradation of the plating adhesion. Furthermore, when the annealing
temperature is lower than 650 C, the surface of the steel sheet is not
activated
and the reactivity between molten metal and the steel sheet is not necessarily
sufficient, while when it exceeds 850 C, surface oxides of Si and Mn are again
formed on the surface of the steel sheet to form non-plated portions. As to
the
atmosphere, the reducing atmosphere is not necessarily maintained over the
whole step, and there may be taken a system that a stage of heating the steel
sheet to 400-650 C is rendered into an oxidizing atmosphere and only the
temperature range exceeding the above is rendered into the reducing
atmosphere.
Further, as the reducing atmosphere, nitrogen gas, argon gas, hydrogen gas and
carbon monoxide gas may be used alone or in an admixture of two or more gases.
Moreover, a temperature history of the heating before plating is pre-
ferable to be a pattern that the temperature is kept at 700-800 C for 0-180
seconds
and then cooled at a rate of about 1-100 C/s.
In the heating before plating, it is not required to control mechanical
properties, and it is enough to heat an original plating sheet to a required
temper-
ature prior to a hot dipping. However, it need hardly be said that the control
of
the mechanical properties may be conducted by the heating before plating.
Hot dipping
In the invention, a hot dipping is carried out on the way of dropping
temperature from the above heating before plating. The method of this hot
dipping is not particularly limited, but may be conducted according to the
conventionally well-known methods.
For example, in case of a galvanization, hot dipping is carried out by
immersing the steel sheet heated before plating in a zinc hot dipping bath
having
a bath temperature of about 460-490 C. In this case, a sheet temperature
inserting into the bath is favorable to be about 460-500 C.
The steel sheet immersed in the zinc hot dipping bath is taken up from
the bath and thereafter subjected to a gas wiping treatment to adjust a
coating
weight to thereby obtain a galvanized steel sheet.
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Further, the galvanized steel sheet may be subjected to a subsequent
hot alloying treatment to obtain an alloyed galvanized steel sheet.
Moreover, there are an aluminum hot dipping, a zinc-aluminum hot
dipping, a zinc-aluminum-magnesium hot dipping and the like as the other hot
dipping treatment, which may be carried out according to the conventionally
well-known methods.
And also, the coating weight in the hot dipping is favorable to be
about 20-100 g/m2 per one-side surface.
BEST MODE FOR CARRYING OUT THE INVENTION
Example I
Slabs having various compositions shown in Table 1 are heated to
1200 C and hot rolled under a condition of finish rolling temperature: 850-900
C.
Then, each of the hot rolled steel bands is pickled and thereafter cold rolled
at a
rolling reduction of 77% to obtain a cold rolled steel sheet having a
thickness of
0.7 mm, which is further subjected to treatments at steps of recrystallization
annealing - pickling - heating before plating - hot dipping using CAL and CGL
under conditions shown in Table 2. Moreover, as an atmosphere gas, there are
used (7 vol% H2 + N2) gas in the recrystallization annealing and (5 vol% H2 +
N2) gas in the heating before plating. Particularly, the heating before
plating in
No. 12 is carried out up to 600 C in a burning gas atmosphere containing 1
vol%
of oxygen and in (10 vol% H2 + N2) gas atmosphere above 600 C.
Galvanizing conditions
bath temperature: 470 C
sheet temperature inserted: 470 C
Al content: 0.14 mass%
coating weight: 50 g/m2 (per one side surface)
dipping time: 1 second
100 specimens having a size of 40 mm x 80 mm are taken out from each of
the thus obtained galvanized steel sheets, from which a specimen(s) observing
at
least one non-plated portion of not less than 1 mm in diameter is as a
rejection.
In Table 2 is shown an acceptable ratio measured from a ratio of
acceptable number.
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CA 02715303 2010-09-21
01829 (PCT/JPO1/07846)
> >
- 0 V 0 U 0
U Q U U
O O Oi O ,-+ rr p
O O - O N
Z O O C. CD 0 C. O C.
C. O O O O O
O 0 0 0 0 = C
O O O O O O
m W) 0 0 0 Cr)
p., 0 0 0
0 0 o O O o O
O C C a c _
o 0 0
N Cr) 00 00 N O, 00 00
b N N 0 --( N - r-+ .
O 0 0 O O O O O
O O O O O O O O
N
W) c,4
H o - I I I I It 00 I I
of 0
0
N V~ N r-+ r-+ O1 V') --~
0 Z O O O O O C. O C.
U 0 0 O O O 0 0 0
O
O O O O
z II = = = I
0 O O
N - - - - I N
U o - - - - O
- in O V) O - 00
N N r-+ N O
O %.DI V1
O -~ O O O
U C = o O = = - 0
O O O O
' Q as U Q W w C7 x
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CA 02715303 2010-09-21
01829 (PCT/JP01/07846)
Table 2
Recrystallization Annealing before
annealing plating Acceptable
Steel ratio of
No. dew Pickling dew Remarks
symbol temperature ( C) temperature ( C) plating
x time (s) point time (s) point (%)
1 A 850 x 60 -30 Condition 750 x 40 -40 63 Comparative
1 Example 1
2 B 100 Invention
Example 1
3 C -10 Condition -50 92 Invention
2 Example 2
4 D -30 Condition -40 34 Comparative
1 Example 2
E 47 Comparative
Example 3
6 F 100 Invention
Example 3
7 B none - none 0 Comparative
Example 4
Condition 880 x 40 Comparative
8 B 800 x 60 -30 I 23 Example 5
9 F 860 x 60 -40 Condition 700 x 40 -45 91 Invention
2 Example 4
G -30 Condition -40 100 Invention
1 Example 5
11 H 850 x 60 0 Comparative
Example 6
12* B 750 x 40 100 Invention
Example 6
Condition 1: 5% hydrochloric acid, 60 C, immersion of 5 seconds
Condition 2: 10% hydrochloric acid, 70 C, immersion of 10 seconds
* Annealing before plating: in a burning gas atmosphere containing 1 vol% of
oxygen up to 600 C and in (10 vol% H2 + N2) gas atmosphere above 600 C
As seen from Table 2, all invention examples have a good plating
property as compared with the comparative examples.
Although an alloying treatment is carried out at 490 C for 60 seconds
in the invention examples 1 and 3, the occurrence of alloyed unevenness is not
observed.
Example 2
Slabs having various compositions shown in Table 3 are heated to
1200 C and thereafter hot rolled at a finish rolling temperature of 850-900 C
to
obtain hot rolled steel sheets having various thicknesses and then pickled.
Then,
they are cold rolled at a rolling reduction of 50-68% to obtain cold rolled
steel
sheets having a thickness of 1.2 mm and subjected to treatments at steps of
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recrystallization annealing - pickling - heating before plating - hot dipping
under conditions shown in Table 4 and described below. Particularly, in No. 24
(steel R), the hot rolled steel sheet (thickness: 1.5 mm) is pickled and
subjected
to treatments at steps of recrystallization annealing - pickling - heating
before
plating - hot dipping without cold rolling.
Moreover, as an atmosphere gas are used (7 vol% H2 + N2) gas in the
recrystallization annealing and (5 vol% H2 + N2) gas in the heating before
plating.
Particularly, the heating before plating in No. 25 is carried out up to 600 C
in a
burning gas atmosphere containing 1 vol% of oxygen and in (10 vol% H2 + N2)
gas atmosphere above 600 C.
Galvanizing conditions
bath temperature: 470 C
sheet temperature inserted: 470 C
Al content: 0.14 mass%
coating weight: 50 g/m2 (per one side surface)
dipping time: 1 second
specimens having a size of 40 mm x 80 mm are taken out from each of
the thus obtained galvanized steel sheets, from which a specimen(s) observing
at
least one non-plated portion of not less than 1 mm in diameter is as a
rejection.
In Table 4 is shown an acceptable ratio measured from a ratio of
acceptable number.
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CA 02715303 2010-09-21
01829 (PCT/JPO1/07846)
> > a) > a)
C
~ ~ ~
U
U U
U d u U
V "I N N MI o0
00
~ O
iJ I I I I I I I I
O O O O O N O O 0
N
z o O 0 0 o, 00, 0 00, 0 0
0 0 0 0 0 0 o O o
o, 0 o o
v) 0 0o 0
o O o o O O O o 0 0
O
C o
M
a) O
o I I I I o I I I I
N
o ~, I I o o I I I I I
p 0 0 0
o - N
0 0
0
z
U o o o 0 0
OI O O o O O O O O
I I O O O O O O O O
'" - ~ I I I I
z II 0 0 0 0
cl!
U II 0 I O I I I I I
'/ O O O N O d O
-+ N N N N e .- N - N
.. N N O N v~ N O o0 00 N
O o O O
O 110 O - O O O O O 00
o
U
o O O O O O O o O O
.~ O
- x- z 0 oa x
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CA 02715303 2010-09-21
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Table 4
Recrystallization Annealing before
annealing plating Acceptable
No. symbol temperature ( C) dew Pickling temperature ( C) dew ratio of Remarks
%
x time (s) point x time (s) point plating
Condition Comparative
13 I 900 x 60 -30 1 750 x 40 -40 30 Example 7
14 J 100 Invention
Example 7
15 none 0 Comparative
Example 8
16 none none none 0 Comparative
Example 9
17 K 900 x 60 -30 Condition 700 x 40 -45 90 Invention
2 Example 8
18 L Condition 750 x 40 -40 10 Comparative
1 Example 10
19 M 850 x 60 100 Invention
Example 9
20 N 100 Comparative
Example 11
21 O 0 Comparative
Example 12
22 P -25 Condition 700 x 40 -45 100 Invention
2 Example 10
23 Q 800 x 60 -30 -40 90 Invention
Example 11
24** R 850 x 60 Condition 80 Invention
1 Example 12
25* P 750 x 40 -30 100 Invention
Example 13
Condition 1: 5% hydrochloric acid, 60 C, immersion of 5 seconds
Condition 2: 10% hydrochloric acid, 70 C, immersion of 5 seconds
* Annealing before plating: in a burning gas atmosphere containing I vol%
of oxygen up to 600 C and in (10 vol% H2 + N2) gas atmosphere above
600 C
** Hot rolled steel sheet (thickness: 1.5 mm) is subjected to treatments of
(recrystallization annealing - pickling - heating before plating - hot
dipping).
As seen from Table 4, all invention examples have a good plating
property as compared with the comparative examples.
Although an alloying treatment is carried out at 490 C for 60 seconds
in the invention examples 7 and 9, the occurrence of alloyed unevenness is not
observed.
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INDUSTRIAL APPLICABILITY
According to the invention, there can be provided various hot-dipped
sheets inclusive of galvanized steel sheets having a high tensile strength and
causing substantially no formation of non-plated portion.
And also, the invention is made possible to provide galvanized steel
sheets having a good alloying property.
Therefore, it is said that the invention considerably contributes to
weight reduction and low fuel consumption of automobiles.
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